1. Field of the Disclosure
The present disclosure is related to magnetic measurements obtained in a borehole and, in particular, to determining a residual measurement error in measurements obtained using a magnetometer or an accelerometer in a borehole.
2. Description of the Related Art
Petroleum exploration generally includes conveying sensors into the well bore for characterizing the down hole formations. The sensor data is used to create various logs recording the properties of the formation. These logs may be obtained either using a measurement-while-drilling process (MWD) a measurement-after-drilling (MAD) process in which sensors are conveyed on the drill string, or using an after drilling process in which sensors are lowered into the wellbore on cable, pipe strings or tubing (these last three are generally described by wireline to differentiate from MWD and MAD). Sensors such as accelerometers and magnetometers are usually included in the suite of sensors, and are generally used for determining the position of the sensors within the wellbore regardless of the conveyance method. These accelerometers and magnetometers sensors contain measurement errors that are intrinsic to the sensor itself, and in the case of magnetometer may be due magnetic sources within the conveyance system, including sensor housings. Although accelerometer and magnetometer measurements are useful, and in many cases necessary, for logging operations, each accelerometer and magnetometer includes various measurement errors that affect its measurements. These errors may be due, for example, to bias (or offset), misalignment, different axial locations and different sensitivities of the accelerometer and magnetometer components. Therefore, there is a need to reduce measurement errors in the down hole sensors measurements, such as magnetometer and accelerometer measurements.
Accelerometers and magnetometers are also used to detect both gravity and magnetic anomalies. Given that locally, the gravity and magnetic fields are virtually constant, these anomalies are generally defined as inclinations from a standard field model, e.g. gravity and WGS 84 gravity model, and the International geomagnetic field model, IGRF, from the United States Geological Survey. Gravity anomalies are generally below the threshold sensitivity of the usual accelerometers used in typical logging system. However, some magnetic anomalies are within the range of sensitivity of the typical magnetometers and if uncorrected generally degrade the magnetometer data. Various formation logs may include accelerometer and magnetic measurements within the formation to determine the formation location within the sub-surface. The magnetometers can detect magnetically anomalous features that may indicate the presence of nearby magnetic material, e.g. a casing, debris in the well, or large scale paleomagnetic effects, e.g. remnant magnetic field in volcanics.